A polygon mirror that is made from a plastic substrate in the shape of a prism and having side surfaces on which a reflection coating is formed is conventionally known in the art.
In an optical reflecting mirror such as a polygon mirror made from a substrate on which a reflection coating is formed, it is required that light should be excellently reflected by the surfaces on which the reflection coating is formed, and further that sufficiently high adhesion be achieved between the substrate and the reflection coating.
In view of the above, it would be desirable to provide a polygon mirror and a reflecting mirror in which not only light is excellently reflected but also the adhesion between the substrate and the reflection coating can be improved, and to provide a method for manufacturing such a polygon mirror.